Seismic surveying is used for identifying subterranean elements, such as hydrocarbons, fresh water, and so forth. In performing seismic surveying, seismic sources are placed at various locations on an earth surface or sea floor (or in a wellbore), with the seismic sources activated to generate seismic waves directed into a subterranean structure. Examples of seismic sources include explosives, air guns, or other sources that generate seismic (acoustic) waves. In a marine environment, seismic sources and sensors can be towed in water by a sea vessel.
The seismic waves generated by a seismic source travel into the subterranean structure, with a portion of the seismic waves reflected back to the surface (earth surface, sea floor, or wellbore surface) for receipt by seismic sensors (e.g., geophones). These seismic sensors produce signals that represent detected seismic waves. Signals from the seismic sensors are processed to yield information about the content and characteristic of the subterranean structure.
As seismic waves travel through an earth formation, the seismic waves are subject to dissipation due to conversion of the energy of the seismic waves into heat by the earth formation. A seismic wave typically has multiple frequencies. The dissipation effect varies at different frequencies, with higher dissipation occurring at higher frequencies of the seismic wave. As a result, the seismic waves lose more amplitude at higher frequencies than at lower frequencies.
Conventionally, when processing measured seismic data, an inverse filtering technique is applied to correct the dissipation (absorption) effects. Typically, the inverse filter increases amplitudes of seismic waves at higher frequencies to counter the dissipation effect.
There are two types of seismic waves: P-wave (also referred to as a compression wave, which extends in the direction of propagation of the seismic wave); and S-wave (also referred to as a shear wave that extends in a direction generally perpendicular to the direction of propagation of the seismic wave). The dissipation (absorption) effect of a seismic wave is represented by an absorption parameter Q, also referred to as a seismic quality factor. Conventional techniques for estimating absorption parameters typically compute pure-wave (or single-mode) absorption parameters; in other words, a P-wave absorption parameter Q is computed based on a P-wave reflected from a subterranean structure in response to a source P-wave; and an S-wave absorption parameter Q is estimated from an S-wave reflected from a subterranean structure in response to a source S-wave. Techniques for surveying subterranean structures based on estimating pure wave absorption Qs suffer from reduced accuracy, since the surveying does not take into account all available data.